1. Technical Field
Various embodiments of the present disclosure relate to semiconductor light-emitting devices and, more particularly, to a method of selective photo-enhanced wet oxidation for nitride layer regrowth on substrates.
2. Description of the Related Art
Light-emitting diodes (LEDs) are a type of semiconductor-based light source. An LED typically includes semiconducting materials doped with impurities to create a p-n junction. The wavelength of the light emitted, and thus its color, depends on the bandgap energy of the materials forming the p-n junction. In silicon or germanium diodes, the electrons and holes recombined by a non-radiative transition produce no optical emission, because these are indirect bandgap materials. The materials used for the LED have a direct bandgap with energies corresponding to near-infrared, visible or near-ultraviolet light. LEDs are usually built on an n-type substrate with an electrode attached to the p-type layer deposited on its surface. LEDs built on p-type substrates, while less common, are available as well. Many commercial LEDs, especially GaN/InGaN, also use sapphire substrate.
Blue LEDs are based on the wide bandgap III-nitride materials such as, for example, GaN (gallium nitride) and InGaN (indium gallium nitride). Blue LEDs typically have an active region consisting of one or more InGaN quantum wells sandwiched between thicker layers of GaN, called cladding layers. By varying the relative InN—GaN molar fraction in the InGaN quantum wells, the light emission can be varied from violet to amber. AlGaN (aluminum gallium nitride) of varying AlN molar fraction can be used to manufacture the cladding and quantum well layers for ultraviolet LEDs, but these devices have not yet reached the level of efficiency and technological maturity of the InGaN—GaN blue/green devices. If the active quantum well layers are GaN, instead of alloyed InGaN or AlGaN, the device will emit near-ultraviolet light with wavelengths around 350-370 nm. Green LEDs manufactured from the InGaN—GaN system are far more efficient and brighter than green LEDs produced with non-nitride material systems.
Advancements have been achieved in recent years to increase the external quantum efficiency of LEDs. Among the various techniques, one known approach is to form a plurality of protrusions on a surface of a C plane (0001) sapphire substrate in a two-dimensionally repeated pattern, and then epitaxially grow a number of GaN-based semiconductor layers on the sapphire substrate. The repeated pattern has a pitch greater than or equal to λ/4 and less than or equal to 20 μm, and side surfaces of the protrusions have an inclined angle that is not less than 90° and not more than 160°. Accordingly, the external quantum efficiency of LEDs thus formed is said to be increased as a result of an optical beam diffraction mechanism due to the two-dimensional pattern of protrusions on the sapphire substrate.
With respect to forming a number of protrusions on a surface of a sapphire substrate in a two-dimensionally repeated pattern, a number of approaches have been proposed but each is not without disadvantages. For example, one approach aligns a SiO2 stripe mask parallel to the [11 2 0] direction of GaN to produce optically smooth layers on the inclined semi-polar {1 1 01} GaN facets. However, impurity migration from the mask region (SiO2) can cause serious material contamination issues to the growth of nitride layers thereafter. Another approach aligns the SiO2 stripe mask parallel to the [1 1 00] direction of GaN to produce optically smooth layers on the inclined semi-polar {11 2 2} GaN facets. However, as with the aforementioned approach, impurity migration from the SiO2 mask region can cause serious material contamination issues to the growth of nitride layers thereafter.
On the other hand, V-notch grooves may be found in an LED structure comprising adjacent layers of Al(Ga)N/In(Ga)N with high bandgap energy and low bandgap energy. Screw dislocations, which are undesirable, tend to exist at the bottom of the V-notch grooves. Etch pit density (EPD) is a measure for the quality of semiconductor wafers. An etch solution, such as molten KOH at 450° C., is applied on the surface of the wafer where the etch rate is increased at dislocations, such as screw dislocations, of the crystal resulting in pits. To suppress further propagation of screw dislocations, one approach utilizes a photolithographic method with alignment procedures which require additional time and costs in operation and equipment. Other approaches require SiO2 masking or dry etch. However, such techniques may result in undesirable impurity migration and/or damage to the crystal atomic structure.